Delft University of Technology

Revealing operando transformation dynamics in individual Li-ion electrode crystallitesusing X-Ray Microbeam Diffraction

van Hulzen, Martijn; Ooms, Frans; Wright, Jonathan P.; Wagemaker, Marnix

DOI10.3389/fenrg.2018.00059Publication date2018Document VersionFinal published versionPublished inFrontiers in Energy Research

Citation (APA)van Hulzen, M., Ooms, F., Wright, J. P., & Wagemaker, M. (2018). Revealing operando transformationdynamics in individual Li-ion electrode crystallites using X-Ray Microbeam Diffraction. Frontiers in EnergyResearch, 6, [59]. https://doi.org/10.3389/fenrg.2018.00059

Important noteTo cite this publication, please use the final published version (if applicable).Please check the document version above.

CopyrightOther than for strictly personal use, it is not permitted to download, forward or distribute the text or part of it, without the consentof the author(s) and/or copyright holder(s), unless the work is under an open content license such as Creative Commons.

Takedown policyPlease contact us and provide details if you believe this document breaches copyrights.We will remove access to the work immediately and investigate your claim.

This work is downloaded from Delft University of Technology.For technical reasons the number of authors shown on this cover page is limited to a maximum of 10.

ORIGINAL RESEARCHpublished: 04 July 2018

doi: 10.3389/fenrg.2018.00059

Frontiers in Energy Research | www.frontiersin.org 1 July 2018 | Volume 6 | Article 59

Edited by:

Neeraj Sharma,

University of New South Wales,

Australia

Reviewed by:

Matthew Ryan Rowles,

Curtin University, Australia

Jianping Huang,

Stony Brook University, United States

Wesley M. Dose,

Argonne National Laboratory (DOE),

United States

*Correspondence:

Marnix Wagemaker

m.wagemaker@tudelft.nl

Specialty section:

This article was submitted to

Energy Storage,

a section of the journal

Frontiers in Energy Research

Received: 04 April 2018

Accepted: 05 June 2018

Published: 04 July 2018

Citation:

van Hulzen M, Ooms FGB, Wright JP

and Wagemaker M (2018) Revealing

Operando Transformation Dynamics in

Individual Li-ion Electrode Crystallites

Using X-Ray Microbeam Diffraction.

Front. Energy Res. 6:59.

doi: 10.3389/fenrg.2018.00059

Revealing Operando TransformationDynamics in Individual Li-ionElectrode Crystallites Using X-RayMicrobeam DiffractionMartijn van Hulzen 1, Frans G. B. Ooms 1, Jonathan P. Wright 2 and Marnix Wagemaker 1*

1Department of Radiation Science and Technology, Delft University of Technology, Delft, Netherlands, 2 ID11 - Materials

Science Beamline, European Synchrotron Radiation Facility, Grenoble, France

For the development of next-generation batteries it is important to understand

the structural changes in electrodes under realistic non-equilibrium conditions. With

microbeam X-ray diffraction it is possible to probe many individual electrode grains

concurrently under non-equilibrium conditions in realistic battery systems. This makes

it possible to capture phase transformation behavior that is difficult or even impossible

with powder diffraction. By decreasing the X-ray beam size, the diffraction powder rings

fall apart in the (hkl) reflections belonging to individual electrode crystallites. Monitoring

these reflections during (dis)charging provides direct insight in the transformation

mechanism and kinetics of individual crystallite grains. Here operando microbeam

diffraction is applied on two different cathode materials, LiFePO4 (LFP) displaying a

first-order phase transformation and LiNi1/3Co1/3Mn1/3O2 (NCM) displaying a solid

solution transformation. For LFP four different phase transformation mechanisms are

distinguished within a single crystallite: (1) A first-order phase transformation without

phase coexistence, (2) with phase coexistence, (3) a homogeneous solid solution

phase transformation and (4) an inhomogeneous solid solution crystal transformation,

whereas for NCM only type (3) is observed. From the phase transformation times

of individual crystallites, the local current density is determined as well as the active

particle fractions during (dis)charge. For LFP the active particle fraction increases with

higher cycling rates. At low cycling rates the active particle fraction in NCM is much

larger compared to LFP which appears to be related to the nature of the phase

transition. In particular for LFP the grains are observed to rotate during (dis)charging,

which can be quantified by microbeam diffraction. It brings forward the mechanical

working of the electrodes due to the volumetric changes of the electrode material

possibly affecting electronic contacts to the carbon black conducting matrix. These

results demonstrate the structural information that can be obtained under realistic

non-equilibrium conditions, combining local information on single electrode crystallites,

as well as global information through the observation in many crystallites concurrently.

This provides new and complementary possibilities in operando battery research, which

can contribute to fundamental understanding as well as the development of electrodes

and electrode materials.

Keywords: microbeam X-ray diffraction, operando, transformation kinetics, Li-ion, X-ray diffraction, electrodes,

phase transitions

van Hulzen et al. Operando Crystallite Transformation Dynamics

1. INTRODUCTION

Li-ion batteries have driven the development of today’sportable electronics and electrical vehicles. More generally,next-generation electrochemical storage is expected to be akey technology for our renewable energy future, includinglarge scale introduction of electrical mobility and lifting thedifference between supply and demand of renewable energysources. This puts significant pressure on the development ofbattery chemistries, which requires comprehensive atomic scaleunderstanding of electrochemical processes. A crucial aspectfor many battery chemistries is the formation, decompositionand transformation of electrochemically active crystalline phases,which typically determine a battery’s cycle life, practicalcapacity and kinetic performance. To understand these structuralprocesses, powder diffraction experiments are applied intensively(Harks et al., 2015; Ma et al., 2016). With ex-situ experimentsnon-equilibrium states and processes are not captured, andoften the reactivity of the components during the processtoward ex-situ measurements hinder the observation of the trueequilibrium state. As a consequence there is a strong demandto do operando structural investigations, that is to performdiffraction during battery charging and discharging.

1.1. Operando InvestigationsOperando powder diffraction, both neutron and X-ray (Harkset al., 2015; Ma et al., 2016; Lin et al., 2017; Peterson et al.,2017; Tan et al., 2017) have contributed to the understandingof the solid state reactions taking place in batteries. Powderdiffraction provides average structural properties over a largeensemble of electrode crystallites present in realistic electrodes.This inevitably obscures the transformation mechanism ofindividual electrode crystallites and how these collectivelycontribute to the macroscopic phase transition, which is keyinformation for thoroughly understanding the relation betweenmaterials properties and the performance of battery electrodes.The potentially (dis)charge rate dependent transformationmechanisms of individual crystallites provides insight in theelectrode material cycle life and what limits the (dis)chargerate of the material. The collective behavior of many individualelectrode particles provides insight in how the total batterycurrent is distributed over the individual grains, potentiallycreating detrimental hotspots. An inhomogeneous currentdistribution enhances local electrolyte decomposition andovercharge behavior, making the lifetime shorter and the safetylower (Kerlau et al., 2007;Maire et al., 2008; Katayama et al., 2014;Taminato et al., 2016; Tanida et al., 2016).

1.2. Operando X-Ray MicrobeamDiffractionRecently, operando microbeam diffraction has been introduced,which is able to probe the phase transformation of manyindividual electrode grains in realistic battery systems under non-equilibrium battery conditions (Zhang et al., 2015; Ganapathyet al., 2016; Niwa et al., 2017). By reducing the X-ray beam size tomicron scale, fewer crystallites will be in Bragg condition and asa result diffract. As a consequence, the powder diffraction rings

fall apart in single crystal reflections, each originating from asingle electrode crystallite. Along operando experiments on twotypes of electrodes, (1) LiFePO4 and (2) LiNi1/3Co1/3Mn1/3O2,we demonstrate the principle, data analysis and interpretation,and hence the information that can be extracted from operandomicrobeam diffraction. These experiments reveal various crystaltransformation mechanisms as well as the transformation timesof individual crystallites, occurring concurrently in the manycrystallites that are monitored. In addition this brings forwardthe active particle fraction and also the mechanically inducedrotations of the electrode crystallites in the electrode matrixproviding new insights in the working of electrodes. The non-invasive nature of these operando microbeam experiments,provides the ability to obtain more understanding of thestructural transformations in realistic battery electrodes, that willhelp understanding the fundamental processes that determinethe performance of batteries.

2. MATERIALS AND METHODS

2.1. Cathode PreparationsThe starting material is carbon coated LiFePO4 from Phostechwith an average particle size of 140 nm. Regular LiFePO4

cathodes were prepared through mixing a slurry of LiFePO4,Carbon Black (Super P), PVDF, (polyvinylidene fluoride, Solvay)in NMP (N-methylpyrrolidone), with a mass ratio of the activematerial (LiFePO4), carbon black (SuperP) and binder (PVDF)of 75:15:10. In addition carbonate template electrodes wereprepared to reach high capacities at high rates as reportedrecently (Singh et al., 2013). For the electrodes, 40 wt% NaHCO3

(Aldrich) is added to the electrode slurry mixing, a large partof which is lost by dissolution in the solvent. The final slurrywas casted on carbon coated Aluminium current collectors bydoctor blading. The coatings were dried on a heater plate underair at approximately 155◦C overnight followed by drying undervacuum at around 60◦C for more than 24h. The resultingcoatings were pressed using a roller hand press to enhance theelectronic contact. After mechanical compaction the NaHCO3

templated electrodes were washed in demineralized water thatreacts with the NaHCO3 to form NaOH and gaseous CO2,resulting in a an electrode where the interconnectivity of theporosity in the electrodes is improved as demonstrated previously(Singh et al., 2013). Finally, the electrodes are dried for atleast 3h under vacuum at 100◦C. The results are reported onelectrodes with loading density between 2 and 4 mg/cm2 andwith a thickness of approximately 10–20 µm. For preparingthe LiNi1/3Co1/3Mn1/3O2 cathodes the same method with ratio,80:10:10, of active material, Carbon Black and PVDF were used.No templating was applied.

2.2. Pouch Cell Preparation and TestingThe electrodes were assembled in coffee-bag type cells allassembled under argon atmosphere (< 0.1 ppm O2/H2O). Theelectrodes were separated by glass microfiber filters (Whatman)with a few droplets of 1 mol/liter LiPF6 (EC:DMC 1:1, Novolyte)electrolyte.

Frontiers in Energy Research | www.frontiersin.org 2 July 2018 | Volume 6 | Article 59

van Hulzen et al. Operando Crystallite Transformation Dynamics

2.3. X-Ray Microdiffraction ExperimentAt the European Synchrotron Radiation Facility in Grenoble,France the operando microbeam X-ray experiments wereconducted at beamline ID11. For the LiFePO4 experimentsusing a combination of beryllium and aluminum mirrors amonochromatic X-ray microbeam with an energy of 45 keV(wavelength 0.27552 Å) and a beam size of ∼ 2x3µm wasconfigured. For NCM an energy of 42 keV (wavelength 0.29519Å) and a beam-size of ∼ 8x8µm was used (NCM crystallites are∼ 1 − 2 µm in size). Figure 2 shows a schematic experimentsetup where the X-ray beam transmits through two layersof Al, the cathode material of interest, the solipor separatorsoaked in EC/DMC electrolyte, a disc of Li-metal and twolayers of Cu. All the electrochemical tests were performedgalvanostatically within a voltage window of 4.3 and 2.5 V vs.Li/Li+ for the LiFePO4 electrodes and of 4.3 and 3.0 V vs.

Li/Li+ for the NCM electrodes using an Autolab PGSTAT302Npotentiostat/galvanostat.

With microbeam X-ray diffraction it is possible to followreflections of individual grains over time. Initial beam exposuretests were conducted tomake sure that thematerial, electrode andelectrolyte do not decompose/degrade. As an additional measurethe sample was moved between two points after each omegascan, to spread the beam exposure. In Table 1 an overview of theexperiment details is provided. For the LFP 1C experiment, withC/n denoting the rate at which a full charge or discharge takes nhours, an omega scan from –2.5 to 2.5 degrees was performed,hence the sample was rotated along the z-axis in ten steps ofeach half a degree (see Figure 1 for the experiment geometry).Each rotation step of half a degree takes 5 s, which is also theframe exposure time. As a result the average temporal resolutionfor individual omega scans is 13.5 s and for a complete omega

FIGURE 1 | In the geometry of 2D X-ray diffraction the X-ray beam lies along the x-axis. ω reflects the angle with which the sample is rotated along the z-axis. And η is

the angle along the diffraction ring.

FIGURE 2 | ID11 X-ray synchrotron beamline passing through the various layers of the pouch cell. The microbeam causes the diffraction ring to break up in spots

originating from single electrode grains.

Frontiers in Energy Research | www.frontiersin.org 3 July 2018 | Volume 6 | Article 59

van Hulzen et al. Operando Crystallite Transformation Dynamics

TABLE 1 | Experiment execution details.

Material Rate ω angle range (step) Nr of ω steps Nr of frames (compounded) Exposure time Full cycle duration Beam size

LFP 1C -2.5–2.5 (0.5) 10 1560 (156) 5 s 2 h 2 × 3 µm

LFP 5C -0.5–0.5 (1) 1 400 5 s 24 mins 2 × 3 µm

LFP 10C -0.5–0.5 (1) 1 417 3 s 12 mins 2 ×3 µm

NCM C/4 -3–3 (0.5) 12 4656 (387) 5 s 9 h 8 ×8 µm

For each cycling rate a different combination of exposure time and angle range was used. In the LFP 1C a 10 step ω scan over 5◦ was performed with an exposure time of 5 s per

step. For the 5C and 10C experiment only one rotation step of a full degree was used to be able to capture the faster dynamics. For the short cycles in the 10C experiment only a frame

exposure of 3 s was used. The NCM experiment used a 12 step ω scan over six degrees with 5 s exposure per step.

scan 135 s. By monitoring the intensity of the reflection duringthe omega scan, it is possible to derive the direction and speed ofthe grain rotation, and when the omega scan completely crossesthe reflection of the individual grain, its grain volume can bedetermined, see supplement of Zhang et al. (2015).

2.4. Data ProcessingEach reflection is indexed based on its 22 angle and its positionon the diffraction ring, defined by its η angle along this ring.In addition the size and the total intensity of each reflectionis determined. Figure 3 shows an EDF (ESRF Data Format)frame that was collected during the experiment. It records theintensity of each of the 2,048 × 2,048 pixels of the detector. TheX-ray beam is small enough for the diffraction rings to breakup in individual grain reflections. The Al and Cu layers showup as strong rings in the corners. In Table S1 the reflectionsof a few grains and their properties are listed. To identify andfollow grain reflections over time Matlab R© was used to processthe experiment data consisting of the collected EDF frames foreach experiment. Three steps were followed: (1) finding grainreflections per frame, (2) labeling grain reflections in differentframes and (3) add meta data which includes the number ofreflections, initial, minimum and maximum 22 values. Thereflections gathered in step (1) make it possible to break downthe reflections according to hkl index. In Figure S1 the averagenumber of reflections per frame per hkl is shown. Connectingthe reflections occurring in different frames, step (2), is the mostchallenging to automate. Here the approach was chosen to startwith a reflection from the first frame and to look for reflections insubsequent frames that have the same hkl and fall within a certainη range (a certain section of the diffraction ring). This process isrepeated until all reflections are labeled. The last step is to addmeta data including number of reflections, min/max 22, η and ω

angles. In Table S2 a few labels and their meta data are shown.The integrated intensity of a reflection can be used to derivethe crystal volume (Zhang et al., 2015). After determining theincident flux of photons 80 from the intensity of a powder ringof your material of interest it is possible to directly calculate thecrystal volume from the observed reflection intensity, see Zhanget al. (2015) supplement note 3 for a detailed explanation.

3. RESULTS AND DISCUSSION

Reduction of the X-ray beam size toward micron size makes thatfewer crystallites are in Bragg condition. As a consequence, the

powder diffraction rings fall apart in single crystal reflections,each originating from a single electrode crystallite. By performingmicrobeam X-ray diffraction during battery operation it ispossible to track many individual reflections concurrently duringa charge/discharge cycle. In Figure 4 the number of LiFePO4

(LFP) and FePO4 (FP) crystallites are shown as a function of timeduring a full 1C charge discharge cycle. The decreasing amountof observed LFP domains, and increasing FP domains duringcharge, and vice versa during discharge reflects the expected first-order phase transition of this material upon lithium extractionand insertion. However, to gain insight how this proceeds withinsingle grains we can monitor individual reflections. Here wediscuss experiment data obtained from three LFP experimentswith C-rates 1C, 5C and 10C, and one NCM C/4 experiment.Details on the charge/discharge regime as well as material loadingand Coulombic efficiency for of each of these experiments aregiven in Table S3.

3.1. Transformation MechanicsIn presenting the results it is useful to first look at compoundeddata where frames belonging to the same ω scan are addedup and averaged (see frame numbers in brackets in Table 1).In Figure 5A the (200) reflection of an LFP grain is shownat various states of charge. During charge both LFP and FPphases are visible between t = 18–55 min. A first observationis the stretched shape of the reflection along the diffractionring. Where broadening in the 22 directions relates to thecrystallite domains size and strain, in this case perpendicularto the (200) lattice plain direction, the distribution of intensityover η suggests mosaicity in the a-lattice direction for both theLFP and FP domains. The intensity of the LFP phase reflectiondecreases while at the same time that of the FP phase is growing,clearly representing the well-known phase transition, but in thiscase monitored operando for an individual crystallite electrodeparticle. During this 1C experiment the LFP-FP phase transitionin this specific grain takes around 37 min, 62% of total chargingtime. From the intensity of the reflections it is possible toapproximate the volume of the grain (see 2.4), and therebythe phase-volume transition-rate. This allows us to calculatethe current this individual grain is contributing, and hence thelocal current density. After the complete transformation fromLFP to FP during charging, discharging is initiated at t = 95min. As expected the intensity of the FP reflection weakens, butalso moves along the η angle. After t = 146 min the reflectioncompletely disappears, and the LFP reflection does not reappear.

Frontiers in Energy Research | www.frontiersin.org 4 July 2018 | Volume 6 | Article 59

van Hulzen et al. Operando Crystallite Transformation Dynamics

FIGURE 3 | Sample of an EDF Frame which is a read-out of the detectors 2,048 × 2,048 pixels. The location of each grain reflection is a unique combination of the

polar coordinates η and 22. Using the 22 angle the miller index can be derived. The η angle can be used to find reflections of the same grain in subsequent frames.

FIGURE 4 | Number of LFP and FP reflections of individual grains during a 1C charge/discharge cycle.

In Figure S3, which shows the omega sensitive data for thisreflection, we can see that the FP domain remains within thevisible omega range. This makes it reasonable to assume that the

FP domain is shrinking but at the same that the LFP domain thatis forming is not in Bragg condition (perhaps the phase transitionto LFP causes a rotational shift) and therefore not visible. At

Frontiers in Energy Research | www.frontiersin.org 5 July 2018 | Volume 6 | Article 59

van Hulzen et al. Operando Crystallite Transformation Dynamics

FIGURE 5 | The line charts in the left most column of both figures show from top to bottom the voltage of the battery, the intensity of the grain reflection, the change

in the η angle and the change in the 22 angle for this particular grain reflection. All for a 1C charge/discharge cycle. The dotted line in the η graph corresponds to the

dotted line shown in the 2D plots. The dashed line in the 22 graph corresponds to the dashed line in the 2D plots. (A) Direct view on a (200) reflection of an LFP grain

during a 1C constant-current experiment. At t = 18 min both LFP and FP phases are visible, indicating phase coexistence occurs in this particular grain. The LFP

phase is is visible from t = 0 – 55 min and the FP phase is visible from t = 18 – 146 min. (B) Here a (020) grain reflection is tracked during a 1C experiment. The phase

transition from LFP to FP at the end of charge is not complete as some intensity is still visible for LFP. At the end of discharge clear solid solution behavior is on display

where this grain never fully transitions back to LFP.

first η slightly increases from 47.5 to 48.5 deg (indicating acounter clockwise rotation along the x-axis). At t = 60 min,at the end of constant-current charge, η steadily decreases to45 deg, evidence of a clockwise rotation. In reality the effectiverotation of this grain is a combination of rotations along allthree axis. Especially rotation along the y-axis will bring the grainout of Bragg condition resulting in decreasing intensity, whichis what seems to happen for this particular grain after t = 60min. These observations imply that LiFePO4 grains embeddedin the electrode matrix are rotating during (dis)charging, aswill be quantified and discussed below. In Figure 5B a (020)reflection is shown, representing another LFP grain, again duringa 1C charge/discharge cycle. This grain displays only very limitedrotation along the x-axis (<1◦ in variation for the η angle).However, the steadily increasing intensity suggests that it rotatesalong the y-axis into the Bragg condition. The transformationtoward FP takes only 9 min, between t = 65 – 74 min (15% oftotal charging time) and is not complete as at t = 74 min the LFPphase remains vaguely visible. Upon discharge, starting at t = 98min, the transformation back from FP to LFP is very differentbecause in this case significant intensity appears between thereflections that represent the FP and LFP end-member phases.This implies that the individual grain upon discharge has a

distribution of b-lattice parameters, most likely indicating thatpart of the material has an intermediate Li composition ashas been observed for the average behavior under high rateconditions (Orikasa et al., 2013; Zhang et al., 2014; Li et al.,2017) and in individual crystallites by microbeam diffractionpreviously (Zhang et al., 2015). Surprisingly however, is that thisis observed at 1C rate, where the rate induced solid solutionbehavior has not been observed, and that it occurs on discharge(lithiation) and not on charge. This non-symmetric behavior isin line with the composition dependence of the charge transferrate constants, and lithiation/delithiation behavior of largeindividual crystallites investigated with STXM nano-imaging(Bai et al., 2016). Such behavior will not be observed with powderdiffraction experiments, as the deviating behavior of a few grainswill be masked by the bulk of the transformation. Surprisingly,even 30min after constant-current discharge is succeeded byconstant-voltage discharge (V = 2.5 V), the intensity profile hasbarely changed, indicating that part of this crystallite is stuckin this solid solution state, which typically relaxes away quicklywhen no current is applied (Orikasa et al., 2013; Zhang et al.,2014; Li et al., 2017). The average 22 angle of the reflection at andafter t = 150min is around 5.35◦, well above the equilibrium valueof 5.25◦ for the LFP (020) reflection. Currently there appears

Frontiers in Energy Research | www.frontiersin.org 6 July 2018 | Volume 6 | Article 59

van Hulzen et al. Operando Crystallite Transformation Dynamics

FIGURE 6 | The line charts in the left most column of both figures show from top to bottom the voltage of the battery, the intensity of the grain reflection, the change

in the η angle and the change in the 22 angle for this particular grain reflection. All for a 5C/10C charge/discharge cycle. The dotted line in the η graph corresponds to

the dotted line shown in the 2D plots. The dashed line in the 22 graph corresponds to the dashed line in the 2D plots. (A) The (200) reflection in this 5C experiment

shows significant η movement at the beginning of charge. This continues until LFP-FP phase transition takes place without phase coexistence and within 2 min.

During the transition back from FP to LFP the a-lattice is changing in a homogeneous way although a part of the crystallite remains in the FP phase. The observed

decrease in integrated intensity during the upper voltage hold is likely caused by the grain rotating out of Bragg condition. Since this reflection is seen around the

equator (η = 90 or η = 270) a rotation along the z-axis will remove the Bragg condition. (B) In a 10C experiment we are following a (200) reflection. At t = 5 min there is

clear phase coexistence during the LFP-FP phase transition. At the FP-LFP phase transition the intensity is spread and this grain remains inhomogeneous for the

a-lattice well after discharging has stopped.

no framework to understand this condition, unless the presentsolid solution phase observed represents an extensive diffuseinterface between the end-member phases. Detailed analysis ofthese observations is outside the scope of this work and willbe the subject of follow-up publications. In the 5C and 10Cexperiments in Figure 6 the 22 profile at the end of dischargealso indicates a rate induced solid solution transformation. Inthe 5C experiment however, the FP-LFP crystal transformation(during discharge) is more homogeneous showing no significantintensity of the end-member reflections, which points to a morehomogeneous change in the a-lattice parameter within this singlecrystallite. In line with earlier findings, this demonstrates that rateinduced solid solution transformation takes place in individualgrains (Zhang et al., 2015). Where the integrated intensity ofthe 10C grain is rather constant during the charge dischargecycle, the integrated intensity of the reflection followed in the5C experiment shows a significant drop at the end of chargeduring the constant-voltage regime (V = 4.2). This reflection isobserved at η = 275◦, which is around the equator (η = 90 orη = 270◦). Reflections observed around the equator will move outof Bragg condition when they rotate along ω (the z-axis). In this5C experiment the sample is only rotated for one degree ω. If a

larger ω rotation was used it would have been possible to recordsimilar integrated intensities for neighboring omegas. The smallomega range chosen here is a trade-off against better capturingthe transformation dynamics of grain reflections that do stay inBragg condition.

Where LiFePO4 is known to display a first-order phasetransition upon charge and discharge which is driventoward a complex solid solution reaction a larger currents,LiNi1/3Co1/3Mn1/3O2 (NCM) displays a solid solutiontransformation even near equilibrium conditions. In Figure 7A

the (108) reflection of an individual NCM grain is monitoredduring C/4 charge/discharge experiment. The gradual change inbattery voltage during charge and discharge, reflects the well-known solid solution reaction of NCM. Consistently, the (108)reflection gradually shifts, representing a gradual change in d-spacing of the (108) plane within a single grain. 22 is decreasingin line with the increase in c-lattice parameter upon charging(delithiation) (Yin et al., 2006). Upon discharge, the reflectionmoves reversibly back toward the position associated with thelithiated state. The observed solid solution transformation withina single NCM grain takes the complete charge and dischargetime. This is the case for all transforming grains (see Figure S2),

Frontiers in Energy Research | www.frontiersin.org 7 July 2018 | Volume 6 | Article 59

van Hulzen et al. Operando Crystallite Transformation Dynamics

signifying that the current is distributed equally over all grainsfor this material during the course of the complete cycle. Thisin contrast to the LiFePO4 grains that (de)intercalate for onlypart of the cycle, indicating more localized transformations. InFigure 7B the (110) reflection of another NCM grain consistentlyshows the gradual shift, in this case toward larger 22 anglesupon charging (delithiations), a consequence of the decreasing aand b lattice parameters. Both the intensity and the η angle arestable throughout the cycle which implies that this particulargrain does not rotate along the y and x-axis, respectively.

These results demonstrate that microbeam X-ray diffractioncan be used to distinguish different phase transformationmechanisms taking place in an individual crystallites. LiFePO4

grains display four different mechanisms, and only one typeis observed in NCM grains: (1) A phase transition withoutphase coexistence, LFP-FP in Figure 6A, (2) A phase transitionwith phase coexistence, LFP-FP in both Figures 5A,B, (3) solidsolution phase transition where the grain remains homogeneousin structure, Figure 7 and (4) solid solution phase transitionwith an inhomogeneous crystal lattice, FP-LFP in Figure 5B.With X-ray powder diffraction, it is hard to deconvolute thecontribution of the different phase transition types to theobserved intensity between the end member states, as noted inLiu et al. (2014). Microbeam diffraction enables this distinctionthrough the ability to monitor individual grains, which comparedto microscopic techniques has the advantage that this can bedone for many crystallites at the same time under realistic batteryconditions.

3.2. Transformation KineticsBy following the individual reflections during (dis)charge, asshown in Figures 5–7, of the LiFePO4 and NCM individualcrystallites, their phase transformation rates can be determined.From the intensity it is possible to approximate the volumeof the observed domains, which, in combination with thetransformation rate, results in the local current exposed to asingle electrode grain. Through the average transformation timesof many individual grains the active grain fractions can bedetermined, as well as the average local current density over theactively transforming grains.

This is hard to determine by other techniques, in particularunder operando conditions, and has previously been performedex-situ at for instance 50% state of charge (Li et al., 2014; Bai et al.,2016). Using microbeam diffraction this was recently reportedas a function of charge rate for LiFePO4 (Zhang et al., 2015),as shown in Figure 8. As observed, at low charging rates theactive particle fraction is low. The increasing current, associatedwith the increasing charge rate, is realized by increasing theactive particle fraction, and to a lesser extent by increasingthe average local current density at individual electrode grains.For LFP we determined the active particle fraction for 1C, 5C,and 10C rate experiments by assessing which fraction of theobserved particles was active. This resulted in active particlefractions of approximately 24, 29, and 36% for 1C, 5C, and10C during the constant-current regime, respectively, in goodagreement with the results shown in Figure 8. For the NCMC/4 charging/discharging experiment shown in Figure 7 theaverage transformation time during charge is ∼ 232 min and

during discharge is ∼199 min, resulting in an overall activeparticle fraction of ∼80%, which is much larger than observedfor LiFePO4 at similar (and even higher) charging rates. Inthe first place charge transport of both electrons and Li-ions isexpected to determine the active particle fraction (and hencethe inhomogeneity of the reactions) in an electrode. The chargetransport depends on many factors including the electrodeconfiguration, porosity, thickness, amount and distribution ofconductive additive and the current rate (Kerlau et al., 2007;Maire et al., 2008; Liu et al., 2010; Fleckenstein et al., 2011;Mima et al., 2012; Nishi et al., 2013; Katayama et al., 2014;Taminato et al., 2016). However, another interesting factor isthe nature of the phase transition, either being solid solution,as occurring in NCM, or a first-order phase transition, suchas occurring in LFP at low rates. Compositional differencesin electrodes that display a solid solution reaction should beexpected to be leveled based on the difference in electrochemicalpotential, whereas there is no electrochemical driving force tolevel the difference in local state of (dis)charge in first-order phasetransition electrodes, as observed experimentally (Tanida et al.,2016). As a consequence solid solution materials are expected toresult inmore homogeneous reactions throughout the electrodes,and therefore also in a larger active particle fraction as comparedto first-order phase transition electrode materials. This mayprovide a rational for the larger active particle fraction observedfor NCM as compared to LFP at low charging rates, wherecharge transport induced inhomogeneities are less likely tobe dominant.

3.3. Electrode Grain RotationIn 3.1 we used compounded data where grain data of a completeomega scan is averaged. When looking at the omega sensitivedata it is possible to detect grain rotation along the z-axis.Figure 9A shows the intensity of the LFP (020) reflection asfunction of time and omega. The shift in ω = −2.25 to 0.25implies that this grain is turning around its z-axis by about2.5◦ in total. Also taking into account the change in η thetotal grain rotation can be calculated, the result of which isshown in Figure 9B. (See equation S1 in the Supplement fordetails on how the total grain rotation is estimated). The abilityof microbeam diffraction to monitor the crystallite rotationsduring (dis)charging provides the opportunity to consider themechanical working in electrodes, most likely induced by thevolumetric changes of the grains itself upon lithium insertion andextraction. It has been shown that the local electron transportbetween the electrode particles and the carbon black network canbe rate limiting, strongly depending on the electronic percolation,homogeneity of the electronic network and the electrode particlesize (Awarke et al., 2011; Li et al., 2014). The observed rotationsmay introduce another factor in the electronic transport, as itmay lead to fluctuations in the electronic contact with the carbonblack matrix. The observed grain rotation can not be attributedto the experiment set up (e.g., beam energy transfer) as it is notobserved when the battery is only exposed to the X-ray beamand no current is applied. Furthermore, not all observed grainsrotate during the charge/discharge regime. At this stage verylittle knowledge exists on these phenomena, where microbeamdiffraction may play an important role in the future.

Frontiers in Energy Research | www.frontiersin.org 8 July 2018 | Volume 6 | Article 59

van Hulzen et al. Operando Crystallite Transformation Dynamics

FIGURE 7 | The line charts in the left most column of both figures show from top to bottom the voltage of the battery, the intensity of the grain reflection, the change

in the η angle and the change in the 22 angle for this particular grain reflection. All for a C/4 charge/discharge cycle. The dotted line in the η graph corresponds to the

dotted line shown in the 2D plots. The dashed line in the 22 graph corresponds to the dashed line in the 2D plots. (A) NCM (108) reflection that is active throughout

the charge/discharge cycle. Here the 22 profile reveals the solid solution nature of the material. (B) This (110) reflection also perfectly follows the electrochemistry. The

intensity practically does not change throughout the cycle pointing to a constant crystal volume.

FIGURE 8 | (A) Active fraction and current density of the particles resulting from the average transformation times. (B): Sketch of the rate-dependent transformation

upon charge as follows from the microbeam diffraction experiments (similar upon discharge only with a smaller active particle fraction, see A). This figure is

reproduced from (Zhang et al., 2015) and licensed by CC-BY.

4. CONCLUSION

With microbeam diffraction it is possible to investigate thetransformation behavior of individual electrode grains while theyare being cycled in a realistic battery system. For LiFePO4, afirst-order phase transition material, this is demonstrated byfour different transformation mechanisms from LiFePO4 toFePO4 and vice versa. Tracking individual LiNi1/3Co1/3Mn1/3O2

crystallites displays the expected solid solution transformation

mechanism, revealing that almost all grains actively participateduring the charge/discharge cycle. In comparison with LFPwherea much smaller part of the grains is active, this indicates arelation between the homogeneity of the reaction and the phasetransformation mechanism. The more localized transformationsin LFP electrodes also result in considerable rotations of thegrain, most likely due to the local volumetric changes, which mayinfluence the local electronic contact of the grains. Microbeamdiffraction makes it possible to directly monitor changes in the

Frontiers in Energy Research | www.frontiersin.org 9 July 2018 | Volume 6 | Article 59

van Hulzen et al. Operando Crystallite Transformation Dynamics

FIGURE 9 | (A) Heat map showing the omegas for which this is reflection is visible. Intensity first appears for ω = –2.5. At the end intensity is strongest for ω = –1 but

is visible all the way to ω = 0.5. (B) The omega rotation taken together with the eta movement gives the total grain rotation.

crystal phase volume, derived from the reflection intensity, whichquantifies the local current. This provides insight in how thetotal current is distributed over the electrodes depending onthe (dis)charging conditions, which is an important parameterthat determines the cycle life. Microbeam diffraction is a non-invasive technique, allowing to monitor up to a few hundredgrains concurrently with a temporal resolution of a few seconds.Thereby it offers the possibility to obtain better understanding onthe transformation mechanics and kinetics in Li-ion electrodesand battery electrodes in general.

DATA AVAILABILITY STATEMENT

The dataset generated while analyzing the grain diffraction datacan be found in the 4TU. Centre for Research Data with this link.Here you will find the complete dataset as presented in Tables S1and S2. The grain diffraction data itself, collected during the ESRFexperiments, is not included in this dataset.

AUTHOR CONTRIBUTIONS

MvH and MW wrote the manuscript, MvH processed andanalyzed the data, MvH and FO prepared the electrodes and

the pouch cell batteries, MvH, FO, MW, and JW performed theexperiments at the ESRF beamline.

FUNDING

The research leading to these results has receivedfunding from the European Research Council underthe European Union’s Seventh Framework Programme(FP/2007–2013)/ERC Grant Agreement n. [307161] ofMW.

ACKNOWLEDGMENTS

We acknowledge the European Synchrotron RadiationFacility for provision of synchrotron radiation facilities andthank the beamline staff for assistance in using beamlineID11.

SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be foundonline at: https://www.frontiersin.org/articles/10.3389/fenrg.2018.00059/full#supplementary-material

REFERENCES

Awarke, A., Lauer, S., Pischinger, S., and Wittler, M. (2011). Percolation–tunneling modeling for the study of the electric conductivity inLiFePO4 based Li-ion battery cathodes. J. Power Sources 196, 405–411.doi: 10.1016/j.jpowsour.2010.07.048

Bai, P., Cogswell, D. A., Liu, X., Jin, N., Yu, Y.-s., Tyliszczak, T., et al.(2016). Origin and hysteresis of lithium compositional spatiodynamics within

battery primary particles. Science 353, 566–571. doi: 10.1126/science.aaf4914

Fleckenstein, M., Bohlen, O., Roscher, M. A., and Bäker, B. (2011). Current densityand state of charge inhomogeneities in Li-ion battery cells with LiFePO4 ascathode material due to temperature gradients. J. Power Sources 196, 4769–4778. doi: 10.1016/j.jpowsour.2011.01.043

Ganapathy, S., Heringa, J. R., Anastasaki, M. S., Adams, B. D., van Hulzen,M., Basak, S., et al. (2016). Operando Nano-beam Diffraction to Follow the

Frontiers in Energy Research | www.frontiersin.org 10 July 2018 | Volume 6 | Article 59

van Hulzen et al. Operando Crystallite Transformation Dynamics

Decomposition of Individual Li2O2 Grains in a Non-aqueous Li-O2 Battery.J. Phys. Chem. Lett. 7, 3388–3394. doi: 10.1021/acs.jpclett.6b01368

Harks, P. P., Mulder, F. M., and Notten, P. H. (2015). In situ methods for Li-ionbattery research: a review of recent developments. J. Power Sources 288, 92–105.doi: 10.1016/j.jpowsour.2015.04.084

Katayama, M., Sumiwaka, K., Miyahara, R., Yamashige, H., and Arai, H. (2014).X-ray absorption fine structure imaging of inhomogeneous electrode reactionin LiFePO4 lithium-ion battery cathode. J. Power Sources 269, 994–999.doi: 10.1016/j.jpowsour.2014.03.066

Kerlau, M., Marcinek, M., Srinivasan, V., and Kostecki, R. M. (2007). Studies oflocal degradation phenomena in composite cathodes for lithium-ion batteries.Electrochim. Acta 52, 5422–5429. doi: 10.1016/j.electacta.2007.02.085

Li, Y., Gabaly, F. E., Ferguson, T. R., Smith, R. B., Bartelt, N. C., Sugar, J. D., etal. (2014). Current-induced transition from particle-by-particle to concurrentintercalation in phase-separating battery electrodes.Nat. Mater. 13, 1149–1156.doi: 10.1038/nmat4084

Li, Y., Li, Y., Pei, A., Yan, K., Sun, Y., Wu, C. L., et al. (2017). Atomic structure ofsensitive battery materials and interfaces revealed by cryoelectron microscopy.Science 358, 506–510. doi: 10.1126/science.aam6014

Lin, F., Liu, Y., Yu, X., Cheng, L., Singer, A., Shpyrko, O. G., et al.(2017). Synchrotron X-ray analytical techniques for studying materialselectrochemistry in rechargeable batteries. Chem. Rev. 117, 13123–13186.doi: 10.1021/acs.chemrev.7b00007

Liu, H., Strobridge, F. C., Borkiewicz, O. J., Wiaderek, K. M., Chapman, K. W.,Chupas, P. J., et al. (2014). Capturing metastable structures during high-rate cycling of LiFePO4 nanoparticle electrodes. Science 344, 1451–1452.doi: 10.1126/science.1252817

Liu, J., Kunz, M., Chen, K., Tamura, N., and Richardson, T. J. (2010). Visualizationof charge distribution in a lithium battery electrode. J. Phys. Chem. Lett. 1,2120–2123. doi: 10.1021/jz100634n

Ma, X., Luo, W., Yan, M., He, L., and Mai, L. (2016). In situ characterizationof electrochemical processes in one dimensional nanomaterials for energystorages devices. Nano Energy 24, 165–188. doi: 10.1016/j.nanoen.2016.03.023

Maire, P., Evans, A., Kaiser, H., Scheifele, W., and Novák, P. (2008). Colorimetricdetermination of lithium content in electrodes of lithium-ion batteries. J.Electrochem. Soc. 155, 862–865. doi: 10.1149/1.2979696

Mima, K., Azuma, H., Yamazaki, A., Okuda, C., Ukyo, Y., Sawada, H., et al.(2012). Li distribution characterization in Li-ion batteries positive electrodescontaining LixNi0.8Co0.15Al0.05O2 secondary particles (0.75 ≤ x ≤ 1.0).Nuclear Inst. Methods Phys. Res. B 290, 79–84. doi: 10.1016/j.nimb.2012.08.016

Nishi, T., Nakai, H., and Kita, A. (2013). Visualization of the state-of-chargedistribution in a LiCoO2 cathode by in situ Raman imaging. J. Am. Chem. Soc.

160, 1785–1788. doi: 10.1149/2.061310jesNiwa, H., Shibata, T., Imai, Y., Kimura, S., and Moritomo, Y. (2017). Domain

size of phase-separated NaxCoO2 as investigated by X-ray microdiffraction.Batteries 3, 5. doi: 10.3390/batteries3010005

Orikasa, Y., Maeda, T., Koyama, Y., Murayama, H., Fukuda, K., Tanida, H.,et al. (2013). Direct observation of a metastable crystal phase of LixFePO4

under electrochemical phase transition. J. Am. Chem. Soc. 135, 5497–5500.doi: 10.1021/ja312527x

Peterson, V. K., Auckett, J. E., and Pang, W.-K. (2017). Real-time powder diffraction studies of energy materials under non-equilibrium conditions. IUCrJ 4, 540–554. doi: 10.1107/S2052252517010363

Singh, D. P., Mulder, F. M., Abdelkader, A. M., and Wagemaker, M. (2013). Facilemicro templating LiFePO4 electrodes for high performance Li-ion batteries.Adv. Energy Mater. 3, 572–578. doi: 10.1002/aenm.201200704

Taminato, S., Yonemura, M., Shiotani, S., Kamiyama, T., and Torii, S.(2016). Real-time observations of lithium battery reactions–operando neutrondiffraction analysis during practical operation. Nat. Pub. Group 6:28843.doi: 10.1038/srep28843

Tan, J., Liu, D., Xu, X., and Mai, L. (2017). In situ/operando characterizationtechniques for rechargeable lithium-sulfur batteries: a review. Nanoscale 9,19001–19016. doi: 10.1039/C7NR06819K

Tanida, H., Yamashige, H., Orikasa, Y., Gogyo, Y., Arai, H., Uchimoto, Y.,et al. (2016). Elucidating the Driving Force of Relaxation of ReactionDistribution in LiCoO2 and LiFePO4 Electrodes Using X-ray AbsorptionSpectroscopy. J. Phys. Chem. C 120, 4739–4743. doi: 10.1021/acs.jpcc.5b10210

Yin, S.-C., Rho, Y.-H., Swainson, I., and Nazar, L. F. (2006). X-ray/neutrondiffraction and electrochemical studies of lithium. Chem. Mater. 2, 1901–1910.doi: 10.1021/cm0511769

Zhang, X., Van Hulzen, M., Singh, D. P., Brownrigg, A., Wright, J. P., VanDijk, N. H., et al. (2014). Rate-induced solubility and suppression of thefirst-order phase transition in olivine LiFePO4. Nano Lett. 14, 2279–2285.doi: 10.1021/nl404285y

Zhang, X., van Hulzen, M., Singh, D. P., Brownrigg, A., Wright, J. P., vanDijk, N. H., et al. (2015). Direct view on the phase evolution in individualLiFePO4 nanoparticles during Li-ion battery cycling. Nat. Commun. 6:8333.doi: 10.1038/ncomms9333

Conflict of Interest Statement: The authors declare that the research wasconducted in the absence of any commercial or financial relationships that couldbe construed as a potential conflict of interest.

Copyright © 2018 van Hulzen, Ooms, Wright and Wagemaker. This is an open-

access article distributed under the terms of the Creative Commons Attribution

License (CC BY). The use, distribution or reproduction in other forums is permitted,

provided the original author(s) and the copyright owner(s) are credited and that the

original publication in this journal is cited, in accordance with accepted academic

practice. No use, distribution or reproduction is permitted which does not comply

with these terms.

Frontiers in Energy Research | www.frontiersin.org 11 July 2018 | Volume 6 | Article 59